Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y04: Vortex Matter in Superconducting Materials and Devices: Structure, Organization, and DynamicsInvited
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Sponsoring Units: DCMP Chair: Alexei Koshelev, Argonne National Laboratory Room: LACC 151 |
Friday, March 9, 2018 11:15AM - 11:51AM |
Y04.00001: Imaging super-fast vortex dynamics and mapping pinning potential of individual vortices Invited Speaker: Yonathan Anahory Using a novel scanning nanoSQUID-on-tip the displacement trajectories of single driven vortices are imaged with sub-Angstrom spatial resolution revealing the complex internal potential structure of individual pinning centers and the hysteretic switching between cellular vortex trajectories [1]. At higher driving currents super-fast vortex dynamics is found in which individual vortices penetrate into a superconducting film at rates of tens of GHz and move at velocities up to tens of km/s that greatly exceed the pair-breaking speed limit of the Cooper-pair condensate, while preserving their integrity as topological defects [2]. Instead of random motion of repelling vortices, striking formation of vortex channels is observed that undergo cascades of branching instabilities and bifurcations as the field and current are increased. The work offers a new insight into the fundamental physics of vortex pinning and of the microscopic instabilities of vortex resistive state in superconductors at high currents, crucial for many applications. |
Friday, March 9, 2018 11:51AM - 12:27PM |
Y04.00002: STM spectroscopy of vortices in atomic monolayers of lead on Si(111) Invited Speaker: Dimitri Roditchev
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Friday, March 9, 2018 12:27PM - 1:03PM |
Y04.00003: Strong-pinning regimes by spherical inclusions in anisotropic type-II superconductors Invited Speaker: Roland Willa To make superconductors technologically relevant, it is crucial to immobilize flux lines (vortices) threading the material. Capturing the pinning mechanism starting from microscopic interactions of vortices with defects poses a very difficult problem. The theory of strong vortex pinning provides a starting point to address this problem. We revisit the different regimes of strong-pinning theory and investigate them using large-scale numerical solutions of the time-dependent Ginzburg-Landau equations [1]. We explore the magnetic-field dependences of the critical current, jc(B), for superconductors containing spherical inclusions with different sizes and densities. Within a wide range of parameters, the vortex configuration is disordered and features a power-law decay of jc(B) ∝ B-α, where the power index α decreases with the particle density. We find a first-order transition of the pinning ground state towards double-occupancy of defects leading to a non-monotonic pin-breaking force and peak effect. Our results provide a framework for interpretation of pinning properties of real materials and call for further generalization of theory. [1] R. Willa et al., Superc. Sci. Tech. 2017, arXiv:1708.01653 |
Friday, March 9, 2018 1:03PM - 1:39PM |
Y04.00004: Dynamic reorganization and thermal history effects in vortex matter Invited Speaker: Gabriela Pasquini History effects related to plasticity and metastable configurations in the vicinity of an order-disorder transition (ODT) are common features in a variety of systems, among which vortex matter becomes a prototype model. In superconducting materials with randomly distributed weak pinning centers, the stable vortex phase at low temperature and low magnetic fields is an ordered dislocation free Bragg Glass (BG). With increasing field and/or temperature, the system undergoes an ODT to a strongly pinned disordered phase, whose fingerprint is the sudden rise of the effective pinning, known as Peak Effect (PE). Below the PE, a field-cooled vortex lattice (VL) remains trapped in a disordered metastable configuration, but can reach the stable ordered BG with the help of large shaking AC fields. The existence of a transitional region between the ordered and the disordered phases and the origin of thermal and history effects observed in the effective pinning remained controversial issues for more than a decade. |
Friday, March 9, 2018 1:39PM - 2:15PM |
Y04.00005: Nonlinear surface resistance and reduction of dissipation in superconductors in the Meissner state under strong RF fields Invited Speaker: Alexander Gurevich Recent technological advances in superconducting Nb cavities have resulted in the accelerating electric fields up to 50 MV/m at quality factors Q(H) up to 1011 @ 2K and 1-2 GHz. The resonator cavity geometry offers an opportunity to probe such fundamental issues as the lower limits of RF dissipation and surface resistance, and the physics of nonequilibrium superconductivity under strong RF current densities close the depairing limit at which the Meissner state becomes unstable with respect to penetration of vortices. In this talk I will discuss mechanisms of reduction of surface resistance Rs by pairbreaking effects which broaden the gap peaks in the density of states, for instance, by a small density of magnetic impurities or a metallic oxide layer at the surface. Pairbreaking effects caused by strong RF or microwave fields can also result is a significant reduction of the nonlinear surface resistance with the field amplitude, which explains the anomalous increase of Q(H) which has been observed on Ti or N-treated Nb cavities. The theory also shows that the surface resistance can be optimized and even reduced below its value for an ideal surface by engineering an optimum local density of states at the surface by impurity management and by surface nanostructuring of the Nb resonators with a thin metallic layer of multilayers of Nb3Sn, MgB2 or iron pnictides. Addressing the fundamental physics and materials science of a 100 nm thick surface layer of RF field penetration can bring new ways of decreasing the surface resistance by optimizing materials disorder and peaks in the quasiparticle density of states while tuning the properties of materials defects which can turn their behavior from beneficial to benign and to deadly. These issues can also be essential for reducing losses in superconducting microresonators used for quantum computing. |
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